1. Field of the Invention
The present invention relates to hydrophones of the type that utilize an optical indication of acoustic pressure. More particularly, this invention pertains to an omnidirectional fiber optic hydrophone that is substantially insensitive to transverse acceleration forces.
2. Description of the Prior Art
The hydrophone is an instrument for measuring acoustic signals in a fluid body, acting as a transducer for converting acoustic wave pressure into a measurable signal. Such signal can then be processed to identify and locate an object within the fluid body. The frequency of the signal, corresponding to the acoustic wave frequency, may identify, for example, a submarine propeller within the detection range.
Hydrophones of the type that generate an electrical signal in response to detected acoustic signals employ an electrically-active element such as a piezoceramic or poly-vinyl-difluride (PVF.sub.2, an electrically active polymer) to convert the acoustic input to the signal. These devices convert the incident acoustic pressure into a voltage that varies with the magnitude and the frequency of the incident acoustic wave fronts.
The generally high electrical impedance of hydrophones of the electrical type requires preamplifiers located near the transducer (the sensing element) for transmitting the electrical signal to the ship via the data bus, necessitating the supply of electrical power to the preamplifier/transducer assembly. In addition to adding complexity, weight and cost to the outboard system, electrical hydrophone systems are subject to failure when the wet-end subassembly is breached and exposed to a seawater environment. The multiple sensors required for detection and localization of quieter acoustic signals further complicate matters by requiring a larger data bus transmission bandwidth and associated bit error requirement. This results in an increase in the weight of the ship due to the additional sensors, associated preamplifiers and cabling required. Finally, such systems are prone to extraneous electromagnetic pickup and interference.
Fiber optic hydrophone technology offers a number of advantages. Fiber optic systems are electrically passive and do not risk exposure of electronics to an undersea environment. Since the sensor is an optical device, an all-optical telemetry (data bus) system is easily implemented. By employing a completely optical detection and telemetry system, extraneous electromagnetic pickup is eliminated. An increase in telemetry bandwidth (high detection bandwidth and/or increased sensor count per line) follows from the use of optical frequency energy. Additionally, a reduction in weight is realized by the elimination of any need for preamplifiers and associated power supplies.
A common fiber optic hydrophone comprises a flat spiral of optical fiber potted in a planar polyurethane base. This omnidirectional sensor is subject to a significant amount of sensitivity to tranverse acceleration forces as a consequence of the susceptibility of the relatively-flimsy planar device to stressing. The acceleration force effectively contaminates the optical signal detected by the fiber spiral. Since it does not possess acceleration cancelling properties, accelerometers must be mounted to the ship's hull to measure the forces that are then modeled (insofar as possible) out of the data.
The utility of the prior art fiber optic hydrophone of the above-identified type is greatly compromised by the additional hardware and signal processing required to null the significant effects of transverse acceleration. Furthermore, exact nulling of acceleration effects is essentially impossible as the hull-mounted accelerometers are unavoidably located at some distance from the hydrophone itself.